The northern beaches closures come after Bondi and Clovelly Beaches were closed on Tuesday due to red algae, known as Noctiluca scintillans or sea sparkle, which can cause rashes and irritations to the skin and eyes.

Bondi reopened on Tuesday afternoon while Clovelly remained closed on Wednesday.

A spokeswoman for Randwick Council said the algae had dissipated throughout Wednesday afternoon, and expected Clovelly to reopen on Thursday morning.

"The lifeguards are closely monitoring the situation," the spokeswoman said.

The algae on the northern beaches was not the same as that from the eastern suburbs, but a separate bloom of the same type, experts said.

Palm Beach and Whale Beach were already closed due to fears a dead whale would attract sharks.

The Metropolitan Sydney South Coast and Hunter Regional Algal Co-ordinating Committees attributed the algal bloom to an "upwelling of nutrient rich deep ocean water onto the continental shelf"', which was often seen after rainfall near river mouths.